This study is a single site, Phase 1/2b, double-blind, randomized, placebo-controlled, clinical trial in healthy adult volunteers to evaluate the safety, immunogenicity and efficacy of a live attenuated ETEC vaccine, ACE527, with and without a mucosal adjuvant, dmLT. ACE527 comprises three genetically attenuated and engineered strains of E. coli, with antigen profiles covering a wide range of ETEC surface colonization factor antigens (CFA/I, CFA/II [CS1, CS2, CS3] and CFA/IV [CS5, CS6]) and also expressing LT-B, the inactive subunit of LT (ETEC heat labile toxin). LT(R192G/L211A), or dmLT, is a derivative of wild-type enterotoxigenic Escherichia coli heat-labile enterotoxin that has been genetically modified by replacing the arginine at amino acid position 192 with glycine and the leucine at amino acid position 211 with alanine. Approximately 36 volunteers (projected enrollments plus possible alternates) will be enrolled in Part A into each of two separate Cohorts. Approximately 30 volunteers per Cohort will be randomized to receive 10^10 cfu total live dose ACE527 vaccine alone, 10^10 cfu total live dose ACE527 vaccine + 25 µg dmLT, or placebo. The placebo preparation will be CeraVacx, which is being used as the buffer in both vaccine formulations. Cohort 1 and 2 volunteers will receive 10^10 cfu total dose of ACE527, 10^10 cfu total dose of ACE527 with 25 µg dmLT, or placebo at 0, 1, and 2 months. Enrollment and dosing of Cohort 2 will be dependent on an acceptable safety profile of the first dose of Cohort 1, based on evaluation of data up until Day 3 by the Safety Review Committee (SRC). The first immunization of each Cohort will be administered in the Center for Immunization Research (CIR) Inpatient Unit, followed by 72 hours of direct post-immunization observation. A Safety Review Committee (SRC) will meet after the first dose of cohort to determine continuation of volunteer dosing on an outpatient basis, and enrollment of Cohort 2. The SRC will meet again after the first dose of Cohort 2; if after review by the SRC the first dose appears safe and well tolerated, subsequent doses will be administered on an outpatient basis. Safety will be assessed by solicited symptoms/subject memory aid and laboratory evaluations. Adverse events (AE)s will be graded according to standardized criteria. The immunogenicity outcome measures of interest include serum IgG and IgA antibodies by ELISA against all vaccine antigens, cytokine assays, B and T cell memory responses, shedding profile of ACE527, and vaccine specific mucosal IgA responses